The vision requirements for robotic soccer have been examined by different researchers [8, 9]. Systems with on-board and off-board types have appeared in recent years. All have found that the reactiveness of soccer robots requires vision system with a high processing cycle time. However, due to the rich visual input, researchers have found that dedicated processors or even DSPs are often needed [2, 8].
The system we used at RoboCup-97 was surprisingly simple. A framegrabber with framerate transfer from a 3-CCD camera was used as the input. A relatively slow processor (166Mhz Pentium) was at the heart of the system, performing all computation.
The detection mechanism was kept as simple as possible. The RoboCup rules have specified well defined colors for different objects in the field and these were used as the major cue for object detection. The RoboCup rules specify a green color field with white markings at the side. Also, it specifies a yellow or blue colored ping-pong ball on the top of the robots, one color for each team. A single color patch on the robot is not enough to provide orientation information. Thus, an additional pink color patch was added to each robot. The ball is an orange golf ball. These colors can be differentiated in a straightforward manner in color-space.
The set of detected patches are unordered. The detected color patches on the tops of the robots are then matched by their distance. Knowing the constant distance between the team-color and the pink orientation patch, we were able to match patches that are this distance apart. Two distance-matched patches are marked as a robot.
Noise is inherent in all vision system. False detections in the current system are often of a magnitude of 100 spurious detections per frame. The system attempts to eliminate false detection using two different methods. First, color patches of size not matching the ones on the robots are discarded. This technique filters off most ``salt and pepper'' noise. Second, adding the distance matching mechanism describe above, all false detections are eliminated.